1. Field of the Invention
The invention pertains to circuits which include bistable, bipolar transistors.
2. Art Background
Bipolar transistors are considered attractive for use in a variety of electronic circuits because, among other reasons, such transistors are capable of operating at significantly higher speeds than, for example, field effect transistors. On the other hand, bipolar transistors have the drawback that, in use, they require continuous electrical power, leading to significant power dissipation problems.
Included among the circuits in which bipolar transistors are employed are, for example, high speed electronic logic circuits. Such a logic circuit necessarily includes at least two (conventional) bipolar transistors to achieve the two electrical states essential to conventional binary logic. In this regard, it has been recognized that if one could achieve a bistable bipolar transistor, i.e., bipolar transistor capable of exhibiting either of two stable electrical states, then the number of bipolar transistors employed in logic circuits could be reduced by half, and therefore the cost of such circuits could be significantly reduced. Moreover, the power dissipation problems inherent in the use of the bipolar transistors would be correspondingly reduced.
A recent advance in the art of bipolar transistors has resulted in the development of what are now called resonant tunneling bipolar transistors (RTBTs). These new devices are significant because, among other reasons, and under appropriate circumstances, they exhibit two (or more) stable states. That is, an RTBT typically includes a heterojunction, and a corresponding potential barrier, at the emitter-base interface. An RTBT typically also includes at least one quantum well layer sandwiched between two potential barrier layers, within the base. (RTBTs have also been proposed in which the quantum well and the two potential barrier layers are positioned intermediate the emitter and base.) Significantly, the presence of the quantum well results in the presence of one or more quantized, discrete energy states within the quantum well. In addition, as discussed below, the presence of these energy states permits one to achieve control over the flow of electrical current from the emitter to the collector by controlling the emission process.
Assuming the Fermi level, E.sub.F, in the emitter of an RTBT is below the first energy state, E.sub.1, in the quantum well, then increasing the base-emitter voltage, V.sub.BE, reduces the energy difference between E.sub.F and E.sub.1. When the two levels are equal, electrons tunneling from the emitter are injected into the first energy state of the quantum well and undergo resonant tunneling (hence the name RTBT) through the two potential barriers with near unity transmission probability. A further increase in V.sub.BE destroys the resonance, resulting in a transmission probability which is much less than one and equal to the product of the transmission coefficients of the two barriers without the quantum well. If the quantum well is also characterized by a second, higher, discrete energy state, E.sub.2, then a further appropriate increase in V.sub.BE will result in E.sub.F being equal to E.sub.2, i.e., will result in a second resonance condition. As a result, a plot of collector current, I.sub.C, versus V.sub.BE results in a series of peaks corresponding to the discrete energy levels of the quantum well. Over a portion of the interval between the current peaks, I.sub.C decreases with V.sub.BE, and thus the RTBT exhibits negative differential resistance (NDR). The degree of NDR is conventionally defined in terms of the peak-to-valley ratio (PVR) in I.sub.C.
As is now known, connecting an RTBT to a load resistor results in a device which exhibits two (or more) stable electrical states. That is, when the load line associated with the load resistor is superimposed upon the plot of I.sub.C versus V.sub.BE, the two (or more) intersections of the former with the latter, where the latter has a positive slope, define the two (or more) stable states of the device. Consequently, the combination of an RTBT and a load resistor is capable of serving as a single-transistor logic and/or memory circuit.
For many applications, there is a need to achieve bistability in the absence of a load resistor. Thus, those engaged in the development of single-transistor devices have sought devices which exhibit bistability in the absence of a load resistor.